Many types of circuit structures, such as silicon-on-insulator (SOI) structures, include transistor structures with source/drain (S/D) regions outside a gate structure and a channel region below or inside the gate structure. While source/drain regions often include doped semiconductor materials, channel regions generally remain undoped, thus a doped transition region between S/D regions and channel regions is frequently needed in order to lower resistance to electrical carrier flow between the S/D regions and channel regions. However, such doped transition regions ideally do not extend far below the transistor gate structure, so as to reduce undesirable short-channel effects. Controlling the formation of doped transitions regions of transistor structures to balance these issues remains challenging.